It is known that a crystal material such as AMTIR-1, silver halide, silver bromide, KRS-5 or the like can be used to attain frustrated internal reflectance of infrared energy. The infrared radiant energy is injected into the crystal so as to reflect at more than the critical angle. The resulting internal reflectance establishes an evanescent field around the crystal. Material placed in or near contact with the crystal will couple with the evanescent field generated by the frustrated internal reflection. A material having spectral absorption features in the bandwidth of the infrared energy imparts a spectral signature to the infrared energy undergoing frustrated internal reflection by coupling with the evanescent field. Ejecting the infrared energy from the crystal to an infrared detector permits the acquired spectrum to be measured using, for example, a known FT-IR spectrometer such as a Mattson Galaxy 5000.
The issue remains how best to deliver the infrared energy to a sample so that it can experience evanescent field coupling with the material being spectroscopically analyzed. Various elaborate proposals have been advanced. For example, U.S. Pat. No. 4,602,869 to Harrick proposes mounting two mirrors on an arm so as to reflect the infrared energy into a prism. The infrared radiant energy can experience between one and three internal reflections which lead to the infrared energy acquiring the spectral content of the material under analysis. The internal reflection prism cell disclosed by Harrick, however, is quite elaborate in its mechanical construction which adds to the cost of the unit while simultaneously detracting from its ease of use. There is no simple way contemplated for removing the crystal or for moving onto additional measurements. Nevertheless, the solution proposed by Harrick has proven to be one of the more successful spectroscopic accessories on the market due, in part, to its simplicity relative to other comparable accessories.
Another proposal is that presented in U.K. Patent Application GB 2 228 083 in which a crystal is mounted and sealed to a fluid chamber so as to become a part of the wall of the chamber while also contacting the fluid. The infrared energy experiences multiple internal reflections. Forming the crystal integral with the fluid containment vessel has an inherent advantage of simplicity of use in that the spectra of new liquids can be attained simply by emptying the vessel, cleaning the inner face of the crystal and refilling it with the new specimen. The requirement for multiple internal reflections also requires the crystal to be rather large which complicates the construction of the fluid chamber. The concept of this fluid chamber has enjoyed no known commercial acceptance or success.
U.S. Pat. No. 3,177,759 to Wilks, Jr. discloses a particularly simple and powerful geometry for a crystal that achieves frustrated internal reflection. The crystal has a series of ridges in the form of triangular prisms which are aligned in a common direction to form what amounts to a Fresnel lens. Infrared radiant energy enters the crystal through a first face of any of the ridges, experiences a single internal reflection and then exits through a second face of any of the ridges. The resulting prism has minimum weight and can be formed through simple injection molding. This crystal, however, is not mounted in any particular base such that the mechanical complexity of the accessory is not reduced.
There is a need in the art for a simple, efficient and mechanically simplified way to deliver infrared energy to a sample so that it can experience evanescent field coupling with material that being spectroscopically analyzed. There is a need to avoid the mechanical complexity of having a plurality of internal reflections within the crystal while also being able to rapidly change between new samples. And there is a need to attain mechanical simplicity using an optical apparatus that is economical to fabricate.
The present invention meets these and other needs by combining a simple infrared crystal support assembly for delivering the infrared energy to an infrared crystal where it can experience frustrated internal reflectance in which the infrared crystal is formed integrally with a removable crystal mounting plate. The support assembly supports a removable crystal mounting plate so that new measurements can be made by quickly replacing the crystal mounting plate having one crystal with a new crystal mounting plane having another crystal for analysis of a different sample. Preferably, the crystal has the shape of a Fresnel lens of modest size that is easily fabricated. The infrared energy can be delivered to the crystal so that it experiences a single internal reflection which helps to reduce the complexity associated with crystals using a plurality of internal reflections. The crystal can be mounted in its crystal mounting plate to form a surface that is integral with one side of the crystal mounting plate to facilitate rapid cleaning and reuse of the crystal mounting plate.
The foregoing objectives, features and advantages of the present invention, and more, are explained in detail below.